HASTE/SS-FSE is a
single-shot technique. This means that data from all of
k-space is obtained after a single 90º-excitation pulse. This requires very long echo trains, which in modern scanners may number 128, 256, or even higher.

By comparison, "regular" FSE/TSE is a
multi-shot technique. This means that although
k-space is traversed much more quickly than in conventional SE imaging, data from several separate RF-excitations are still needed to acquire all the data. For example, if 128 lines of
k-space were to be sampled, a FSE/TSE sequence with an ETL/Turbo factor of 16 would require 128/16 = 8 "shots" to completely collect all the data.

To minimize the number of lines sampled, HASTE/SS-FSE uses
phase-conjugate symmetry (a
partial Fourier method) that takes advantage of certain "mirror-image" properties of
k-space and the MR signal. This allows only a little more than half of
k-space data to be directly collected, while the remaining lines can be estimated. A typical pulse sequence timing diagram for HASTE/SS-FSE is shown below:

Representative HASTE/SS-FSE pulse sequence. Note asymmetric sampling in phase-encode direction. The peripheral lines on one side
of
k-space are not sampled but estimated by conjugate symmetry.

HASTE/SS-FSE techniques have already found robust applications throughout the body, including: routine scout images; images of the head or body in children or uncooperative patients; fetal imaging; non-breath-hold abdominal imaging: MR cholangiopancreatography; MR myelography, and non-contrast MR angiography. By necessity, echo times are relatively long, so HASTE images are typically T2-weighted. However, in conjunction with preparatory inversion pulses they can be made to assume some degree of T1- and spin-density-weighted contrast. A few representative examples are shown below.